A polyester polyol mentioned in the opening paragraph is known as such from the international patent application published as WO2008/037773-A1. More in particular, this publication discloses a polyester polyol with an ABA-structure, in which the initiator part B originates from a diamine like 4,4′-diaminodiphenylmethane or from a diol like 1,4-butanediol. The length of the lactide oligomer part A in the polyol structure ranges substantially between 20 to 140 lactate unities. Moreover, the length of the oligomer at both ends of the polyol structure is substantially the same. The polyurethane obtained with this polyester polyol appears to show an increased glass temperature and improved mechanical properties.
Applicant has experienced that the known polyurethane shows a toughness which is too low for a number of applications. The toughness of a material is to be understood as the ability of the material to absorb energy and to deform plastically without fracturing. The toughness is a property which can be determined on the basis of the stress-strain curve or the load-elongation curve of a material, and is expressed in MPa.